This study was performed to investigate the effects of n-3 fatty acids on renal function in male Sprague-Dawley rats of different ages 5-, 15- and 19-months old. The rats were fed a 20%(w/w) lipid diet containing 10% fish oil, compared with control animals fed a 20% lipid diet without fish oil for 4 weeks. The results were as follows: kidney weights were significantly higher in fish oil-fed rats compared to control rats. Plasma levels of total lipid, total cholesterol, and triglyceride markedly increased, with aging and LDL-cholesterol showing a significantly lower level in fish oil-fed rats than control rats. The urinary protein and glomerular filtration rate (GFR) increased with aging. GFR was higher in fish oil-fed rats. However, urinary protein was the same in the two groups. Renal medulla thromboxane B$_2$(TXB$_2$)tended to be lower in fish oil-fed 19-month-old rats. Urinary TXB$_2$and PGE$_2$were found to be higher proteinuria. Light microscopic examination showed interstitial inflammation, tubular atrophy, interstitial fibrosis and glomerular mesangium increase. Although glomerular sclerosis increased with aging, fish oil in the diet had no effect on histological changes. In conclusion, plasma lipid, urinary protein excretion and renal histological change showed a significant increase with aging. The reduction of TXB$_2$in the medulla and increase of GFR caused by fish oil indicated n-3 fatty acid could affect renal function in line with the hypolipidemic effect.